ESLint Shared Configuration

Packages maintained by ESL Team often use ESLint to ensure code quality and consistency. To simplify the process of configuring ESLint for these packages, we have developed a shared ESLint configuration. This configuration is designed to be used as a base for ESLint configuration in projects that use ESL package, or decide to follow ESL source code style.

Installation

To use ESL shared ESLint configuration, you need to install it as npm package:

npm install --save-dev @exadel/eslint-config-esl

Ensure that you have the ESLint package of version 9.0.0 or higher, shared configuration is distributed only as flat ESLint config.

Once installed, the configuration needs to be added in eslint configuration file:

// Import only configs you need
import {lang, base, codestyle, medium, strict, esl} from '@exadel/eslint-config-esl';

export default [
    // Default lang (parsers) configs, if required
    //...lang.js,
    //...lang.ts,
        
    // ESL ESLint configuration of your choice
    ...strict, // or medium, or base, or codestyle   
    // Note: recommended sets of eslint / typescript-eslint are included in base, medium and strict config.     

    // (Optional) ESL integrated migration / deprecation checks (warn severity by default)
    ...esl.recommended(),
        
    // ESLint user configuration ...
];

Please note '@exadel/eslint-config-esl' is an ESM-only package. You either need to use ESM in your project or run eslint independently with .mjs configuration file.

Configuration

ESL Shared ESLint Configuration is split into several configurations/bunches:

The base ruleset recommended for consumers to have basic code checks similar to ESL source code. The codestyle ruleset is recommended for projects that want to follow ESL code style. The medium ruleset is a shortcut for projects that want to have both basic and code style checks. The strict ruleset is recommended for projects that completely follow ESL code style and want to have the most restrictive checks.

Default lang (parsers) configs

ESL Shared ESLint Configuration includes default lang (parsers) configs for JavaScript and TypeScript.

Feel free to extend or default them manually.

Default JavaScript parser config:

const js = {  // lang.js
    files: ['**/*.js', '**/*.mjs'],
    languageOptions: {
        ecmaVersion: 2017,
        sourceType: 'module',
        globals: {
            ...globals.browser,
            ...globals.node
        }
    }
};

Default TypeScript parser config:

const ts = { // lang.ts
    files: ['**/*.ts', '**/*.tsx'],
    languageOptions: {
        parser: tsParser, // @typescript-eslint/parser
        ecmaVersion: 2017,
        sourceType: 'module',
        parserOptions: {
            projectService: true
        },
        globals: {
            ...globals.browser
        }
    },
    settings: {
      'import/resolver': {
        typescript: true
      }
    }
};

Inner Plugins

ESL Shared ESLint Configuration includes several inner plugins that are used to provide additional rules and configurations for ESLint. Here is the list of included plugins and their ESLint aliases:

All mentioned plugins are direct dependencies of @exadel/eslint-config-esl package, you don't need to install them separately.

Deprecation & Migration Assistance (Integrated)

The previously separate @exadel/eslint-plugin-esl (migration / deprecation plugin) was removed. Its functionality is now integrated directly into this shared config package under the esl export.

Key points

Enabling (default WARN)

import {esl} from '@exadel/eslint-config-esl';
export default [
  ...esl.recommended(), // same as esl.recommended('warn')
];

Custom severity

Accepted severity values (case-insensitive):

Examples:

...esl.recommended('error');       // escalate
...esl.recommended(0);             // disable all deprecations
...esl.recommended('off');         // same as above

Forcing a specific ESL version (tests / CI / preview)

You can force generation for a target ESL version (bypassing auto-detection):

import {esl} from '@exadel/eslint-config-esl';
export default [
  ...esl.version('6.1.0', 'warn')
];

This is mainly useful in CI or when validating upcoming upgrades.

Verbose output

Run ESLint with --verbose to see a single log line indicating which ESL version (or superset fallback) was used to build the deprecation config.

Migration from removed standalone plugin

Historically a standalone migration / deprecation plugin existed (@exadel/eslint-plugin-esl). It exposed grouped configs separating deprecations introduced in ESL v4 and v5. Those configs (default-4, default-5, default, plus a flat-friendly recommended) are removed and replaced with the version‑aware esl.recommended() helper in this package.

Minimal legacy examples for context:

// .eslintrc.js (legacy non-flat)
module.exports = {
  plugins: ['@exadel/esl'],
  extends: ['plugin:@exadel/esl/default']
};

// eslint.config.mjs (legacy flat)
import deprecated from '@exadel/eslint-plugin-esl';
export default [
  ...deprecated.configs.recommended
];

Other legacy variants included:

Current approach (single spread):

import {esl} from '@exadel/eslint-config-esl';
export default [
  ...esl.recommended('warn')
];

Key changes vs legacy:

Need strict treatment for older deprecations? Use esl.recommended('error') and inline-disable rare new ones temporarily.

Migration checklist:

  1. Remove @exadel/eslint-plugin-esl from devDependencies.
  2. Drop any plugin:@exadel/esl/<config> extends and legacy rule IDs.
  3. Add ...esl.recommended(<severity>) (default warn) to your flat config.
  4. Clean obsolete inline disables referencing removed rule names.
  5. (Optional) Run ESLint with --verbose to confirm detected ESL version.

Edge cases:

Need multi-tier severities back? Open a discussion; future version-scoped preview modes may offer structured staging without reintroducing legacy complexity.

Why a single severity knob?

Exadel, Inc.